Determine your ghost story's demographic. While ghosts stories are often read by children, adults enjoy a good scary ghost tale as well. Decide who your audience is, and what you want to make them feel. Begin to construct ideas that might be suitable---for example, if your audience is teenagers, consider writing about a high-school ghost. Adult ghost tales might have an erotic undertone. Determine what your best writing voice is, and for whom you will write.
Read other ghost stories in similar genres to get an idea of what you want to write. Take note of how other writers illustrate spooky images, build a ghastly plot line and introduce potentially scary characters. If, for instance, you want to write a fresh, edgy teen ghost tale, peruse the public library and take a look at how other common teen ghost story writers---like R.L. Stine---execute their books. Take some notes on how the writer pulls the reader in: Is the ghost bad or good? Is the ghost sympathetic or complex? Are the protagonists struggling to understand the ghost? Take note of the authors' fine detail points. Let this inspire how you will build your story in a way that is unique to you.
Get to know your ghost. Since you're writing a ghost story, the main element of the tale will be the ghost or the effects of the ghost on others. According to Time to Write, a good ghost story should have a few key elements: an obvious ghost character, detailed settings and weather, and a struggle concerning the ghost's desires. Details make a story stand out. Figure out what sort of setting you want to go for (a house, mansion or cemetery are common ghost settings) and what kind of weather sets the mood (storms tend to increase the spooky factor). Determine who your ghost is and what he wants---ghosts usually want revenge or to connect with the living. Write down these details in a notebook and turn to it when writing your story.